The Automatic Identification System (AIS) is a short-range (typically 20-46 nautical miles) tracking system used by ships and terrestrial marine traffic monitoring stations for identifying and locating ships. Each ship has an AIS transponder that automatically broadcasts short messages (up to 256 bits) at regular intervals. These messages may include information about the ship's identity, such as IMO number, call sign and name, UTC time, length, location, course over ground, speed over ground, and other details about the ship, it's movement or cargo. The messages are received by AIS transponders fitted on other ships or land-based monitoring stations. The frequency at which a ship transmits AIS messages is based on the state of the ship's motion. For example, ships that are anchored or are moving slowly transmit less frequently than those that are moving faster or are maneuvering.
AIS messages are typically transmitted on one or both of two Very High Frequency (VHF) channels centered at 161.975 MHz and 162.025 MHz and use 9600 bits/s Gaussian minimum shift keying (GMSK) modulation over 25 or 12.5 kHz channels using the High-level Data Link Control (HDLC) protocol. However, it will be evident to a person of skill in the art that future global and regional AIS systems may make use of additional frequencies. In the usual mode of operation, ship transmissions alternate between the two channels on a per transmission basis.
To ensure that VHF transmissions of different AIS transponders do not occur at the same time, the signals are time multiplexed using a technology called Self-Organizing Time Division Multiple Access (SOTDMA). Under SOTDMA each minute of time is divided into 2,250 equal time slots of 26.67 ms each. With a transmission speed of 9600 bits/s this translates into 256 bits per timeslot.
The time slots are typically synchronized accurately to Coordinated Universal Time (UTC) typically using Global Positioning System (GPS). Each AIS transponder determines its own transmission slot based upon the signal levels of AIS messages received from other ships and knowledge of future actions by other ships. Specifically, slot selection by an AIS transponder is randomized within a defined interval, and tagged with a random timeout of between 0 and 8 frames. When a ship changes its slot assignment, it announces both the new location and the timeout for that location. This self-organizing feature avoids signal collisions over the short ranges involved in surface transmissions and permits many ships to share the AIS frequencies efficiently.
The use of two AIS channels allows for up to 4500 messages per one-minute time frame. This capacity is relevant for a self-organized region determined by the broadcast range of the ship transponders (typically 20-46 nautical miles in radius). Where there are more ships in a particular broadcast range, the transponders will automatically reduce their receive range to reduce the size of the SOTDMA region. Specifically, under the SOTDMA CS protocol used with Class-B AIS transponders, a ship's transponder will start-up with a low signal level threshold of −107 dB. This means that the ship will accept or receive AIS signals with a signal level of −107 dB or greater. If a ship is unable to find an empty slot with this particular low signal level threshold (e.g. there are too many ships in this receive range), then the ship's transponder will incrementally step up the low signal level threshold until the transponder is able to find a free slot, or the low signal level threshold reaches −77 dB. In practice, this threshold is offset by a further 10 dB from the actual measured signal level. Class-A AIS transponders also co-ordinate their transmissions in a similar fashion, using received transmissions from other ships as well as shore-based AIS stations to utilize pre-allocated time slots that assigned to particular ships while within range of the shore-based AIS station. It should be noted that changes to the low signal level threshold do not affect the transmit range, as transmit power level is not changed. In most cases the transmitter power is fixed at about 2 W for Class-B, and 12.5 W for Class-A AIS transponders.
The use of AIS is now mandatory on all ships with a gross tonnage of 300 or more tons engaged on international voyages, and all passenger ships regardless of size or tonnage. It is estimated that more than 40,000 ships carry AIS equipment. AIS was originally conceived as an aid to navigation and safety, and also has potential security applications for monitoring maritime traffic. AIS signal detection could be achieved using coastal/ground stations, but the limited range of the VHF signals would require such coastal/ground stations to be situated at many locations along the coast, and even then, they could only monitor the immediate coastal region.